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Charging two GP Ion-Li Battery packs with only one IC?

Former Member

Hello,

 

I have this two Li-Ion Battery packs from GP Batteries:

 

7.4V

http://es.farnell.com/jsp/search/productdetail.jsp?SKU=1848661

 

11.1V

http://es.farnell.com/jsp/search/productdetail.jsp?SKU=1848662

 

I have put them in series -> 18.5V

I want them to charge with for example LTC4008EGN#PBFLTC4008EGN#PBF from Linear Tech.

http://es.farnell.com/linear-technology/ltc4008egn-pbf/cargador-de-baterias-smd-ssop20/dp/1273983?Ntt=ltc4008

 

Question is, what happens if one of the packs is medium-charged and the other completely discharged? I understand that in a certain moment one of the packs will be charged, but what happens with the other pack?

Will continue charging? Will the charged pack be damaged? Or simply one of the packs will remain without being charged?

 

Is it imperative in this scenario to use one charger for each pack??

 

Thanks for any tip.

 

Regards,

 

Cristian.


Parents Reply
  • 0 in reply to RWM

    image Good point. and very easy implementation.

     

    Ahh I hope so too, the only thing that scares me a bit is the mosfet´s gates. I remember that once in a project of an industrial printer, in the power section with Step-Down Converters, we spent a couple of months trying to know why the mosfets break down when plugging the system. I spent one month totally confused, I remember it like a nightmare, later another engineer and the same... the problem was resolved changing the CC/CC chip... don´t remember the brand... I only remember my desk full of mosfets and dreaming with some fantastic sculpture with all those broken mosfets, jajaj.

     

    I hope LTC4008 to be polite with the mosfets. I´ll try to post the results.

     

    Cheers.

Children
  • 0 in reply to Former Member

    If you will keep tracks from IC to transistor gate short, I think there should not be any problems. As gate drive voltage is 5.6V typically, you should use logic-level MOSFETs.

     

    I wish you success with your design.

     

    All the best!

  • 0 in reply to RWM

    Response from LTC factory applications

     

    Cristian, it seems as if your questions have already been answered, but we’d still like to make some comments. In general, the greater number of batteries in series and greater capacity would require charge balancing. 3-4 series cells and up tends to be a general manufacturer borderline for cell balancing.  Farnell should be able to comment on this. Tenergy would likely say the same. And by the way, Tenergy does make 5-cell packs and can make one-off packs of pretty much any number of series cells. I’m not sure if your application is a low-volume consumer, private hobbyist, etc. Could you advise? We should be able to either suggest a pack for your usage (if you haven’t already chosen one) or get you in touch with the sales guys that could help you develop a one-off 5-cell pack. We have asked for this in the past for reference designs.
     
    As you’ve already considered, if charging two distinct packs of 2 and 3-cells respectively, one could get away with two separate chargers without balancing. Since the batteries are stacked, you & Ryszard have correctly observed that the “top” charger for the upper cells of the stack would need to float in series from the “bottom” charger. You could reference that circuit’s GND from the VBAT+ of the lower circuit, but you would still need an isolated connection to the input side of each charger. It would be necessary to have a front-end isolated topology, such as a dual output winding flyback to take the wall adapter input. This would actually be helpful in terms of providing the lowest VIN possible for highest efficiency to each charger while providing the proper output voltage. For example, the 2-cell charger would only require ~10VIN, while the 3-cell charger would only require ~14VIN. But, your overall system efficiency will hinge on how well the front-end flyback is designed.
     
    For your proposed approach with 5-cells all in one pack, you could certainly still use LTC4008. Again, Ryszard is correct in identifying the difference—or similarity, rather—between LTC4007 & LTC4008. However, we don’t recommend LTC4006/7 for anything beyond 2-4 series cell stacks; LTC4008 has a resistor adjustable output up to 6 cells and beyond. As I said, this approach requires cell balancing and since this is a step-down topology (buck), you’d always need VIN>VBAT, which is ~22VIN for headroom above the 21VBAT. LTC4009CUF-2#PBFLTC4009CUF-2#PBF & LTC4012 (recommended for higher current >4A) are also options, though you would need separate microcontroller connections for timer and thermal-qualified termination.
     
    For Li-ion batteries, the temperature is very stable during the entire charge cycle, but temperature failsafes are required for charging either below or above the manufacturer recommended temperature range. Termination for Li-ion/poly/FePO4 chemistries are all the same: timer or current-threshold (C/x). Most times a C/x termination is sufficient as the remaining time spent charging the battery from ~90-95 up to 100% is almost as long as the first 95%.
  • 0 in reply to Former Member

    Hi Mike,

    You are very wellcome of course, as you said, there is a border-line when 5-cells Li-Ion packs are needed.

    Aeromodelism shops sells 5 cell packs (cant remember the brand) but without any protection or cell balancing. By the way, we can find up to 4-cell packs with cell balancing and protection circuit (OVP) included.

    The only reasonable way I have found for getting 5-cell packs with protection and balance circuit included is directly from Tenergy. Problem is that price goes higher (and shipment from EEUU is expensive too).

     

    This is required for geotecnic applications. We sell pressuremeter probes for Spain that needs 18-24 VDC for operation. What we design is the equipment needed for reading the values, power the probe, computer connection through USB and software. All included in one case with batteries inside, as this is usually used for fieldworks.

    We have jumped from Lead-Acid to Li-Ion recently. Then number of units we sell per year it´s linked to the activity of geotecnic companies here in Spain. So it´s for low-volume consumer of course.

    I would have designed a step-up converter from a 4-cell pack, but I would like to avoid it because of the noise it can spread around the pcb and of course, the efficiency lost.

     

    For the moment the only quality choice I have seen for the 5-cell pack is buying directly to Tenergy or a Tenergy-reseller who has bought a high quantity of this 5-cell Tenergy packs. I remember that Farnell told me that their products availability depends on the demand... I would be glad to hear alternatives for Europe. Thank you very much for your lines.

     

    Best Regards,

     

    Cristian,

    NDT Ingenieros S.L.

    Spain

  • 0 in reply to Former Member

    Hi Cristian,

     

    fo small quantity you can build your own pack using e.g. bq77910 protection/balancing IC:

    http://www.ti.com/product/bq77910